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Prepared by:
Weiping Dai, Ph.D., P.E.
BREEZE Software
12700 Park Central Drive | Suite 2100 | Dallas, TX 75251
+1 (972) 661-8881 | breeze-software.com
Managing Risks Utilizing Air Dispersion Modeling for Offshore Production Industry
Managing Risks Utilizing Air Dispersion Modeling for
Offshore Production Industry
Weiping Dai, Ph.D., P.E.Ventura, CAMay 14, 2004
trinityconsultants.com
AWMA West Coast Section Annual Meeting
Objectives
Understand potential air emissions from offshore oil & gas exploration and production operations.Study available air dispersion modeling tools to manage risks due to air emissions from offshore operations.Demonstrate the proper application of dispersion modeling tools.
Outline
Air Emission Characteristics from Offshore Operations (Pollutants/Release/Hazards)Potential Risks of Air Emissions (Health and Safety Hazards)Air Dispersion Modeling Tools and Applications
Air Emissions from Offshore OperationsProduct Characterization
Crude Oil – mixture of many different hydrocarbon (HC) compounds including natural gas.Natural Gas – associated gas from oil wells or non-associated gas from gas wells
Mixture of methane (dominant), ethane, propane, butane, etc.
Liquefied Natural Gas (LNG)Natural Gas Liquids – Propane and butane liquefied via cooling and compressionAll these products contain other constituents and impurities that could be emitted to the atmosphere
Air Emissions from Offshore OperationsProcesses with Potential Air Emissions
Drilling/Well DevelopmentFugitive natural gas, VOCs, CO2, CO, H2S, etc.
ProductionCrude oil separation - Separation of gaseous components such as natural gas and H2SNatural gas conditioning - Removal of hydrogen sulfide and/or carbon dioxide; fugitive BTEX
Maintenance (workovers) VOCs, paints, HCl (scale removal)
Spills and BlowoutsAccidental release from leaking tanks, flowlines, valves, joints, and gauges, etc. – VOCs and in-site burning productsBlowouts of associated natural gas and fire
Air Emissions from Offshore Operations
Air Emission SourcesFlaring (CO, NOx, SO2, VOC) and Venting
Combustion products from flaring natural gas or acid gas
Exhaust from diesel engines and turbines (NOx, PM, O3, CO, SO2)Fuel combustions from pumps, heater-treaters, and motors
Potential Risks due to Air EmissionsHealth & Safety Concerns
Hydrocarbons/VOCs (benzene, toluene, naphthalene, etc.) –HAP, toxic/fire hazardsHydrogen Sulfide – toxic (and even fatal at certain concentrations) to humans and corrosive for pipesCarbon Dioxide – Greenhouse gas contributing to global warmingGlycols from natural gas processing – volatile and hazardousChlorofluorohydrocarbons (CFCs) – Good fire fighting agents but causing ozone depletion; gradually phasing-outCriteria pollutants: SO2, CO, NOx, PM10, O3
Flaring of combustible/poisonous gases like methane and hydrogen sulfide reduces health and safety risks in the vicinity of the well.
Potential Risks due to Air Emissions
Personnel on-site or at nearby locations may expose to excessive toxic concentration or heat radiation.Damage to on-site or nearby property due to fires caused by accidental release.Adverse impact on ambient air quality.
Air Regulatory Requirements for Offshore Operations
Clean Air Act – covers coastal areas and the offshore regions of the Pacific, Atlantic, Arctic Oceans, and region of the Gulf of Mexico adjacent to Florida
National Emission Standards for Hazardous Air Pollutants (NESHAP)New Source Performance Standards (NSPS)
Air Regulatory Requirements for Offshore Operations
Minerals Management Service (MMS) Air Quality Standards (30 CFR Part 250) –covers Gulf of Mexico adjacent to Texas, Louisiana, Mississippi, and Alabama.
Limits for VOC, CO, NO2, SO2, and TSPBlowout prevention regulationsVenting and flaring of natural gas
Why, or When, to Model?Modeling typically conducted for one of the following reasons
Regulatory RequirementsAir Quality StandardsToxic air pollutantsDevelopment of mitigation strategies
Engineering AssessmentsSite planningEmissions control
Hazardous ReleasesEmergency response planning and operationsRisk assessment
Air Dispersion Modeling ToolsCriteria to select proper dispersion model:
Continuous Release (> 1 hr) vs. Short-duration Accidental Release (< 1 hr)Neutrally Buoyant vs. Dense GasElevated Source or Ground-level SourceGas/Aerosol (two-phase accidental release)Toxic or Fire Hazards
Major Types of ModelsGaussian Models for continuous releasesShort-term accidental release models
Structure of a Dispersion ModelFor Each Source
Physical HeightPollutant Emission RateCoordinatesStack DiameterStack Gas VelocityStack Gas TemperatureDimensions Used to
Characterize Building Wake Effects
MeteorologyPasquill Stability ClassWind DirectionMixing HeightAmbient TemperatureWind Speed
For Each ReceptorCoordinatesGroundlevel ElevationHeight Above Ground
Simulation of Atmospheric Physics
Estimate of Air PollutantConcentrations at Receptors
Gaussian Dispersion Models
Suitable for steady-state continuous emissions from
FlaringVentingFugitive Emissions
Example Models: ISCST3, SCREEN3, OCD, AERMOD, ISC-PRIME, CALPUFF.
Ambient Concentration Calculation with Gaussian Models
The concentration at the receptor at x, y, z from a source with effective height, H is:
Emissions Q
Downwind factor 1u
Crosswind factor
2
2yy
1 yexp2 σσ2 π
−
Vertical factor 2 2
2 2z zz
1 (H z) (H z)exp exp2 σ 2 σσ2 π
− + − + −
χ( , , : )x y z H =
Ambient Concentration Calculation with Gaussian Models
Ambient concentration is a function of emissions, downwind, lateral, and relative vertical distance from the source, cross-wise distance from the flow direction, wind speed, and PGT stability class
Any effects on plume behavior have to be parameterized in terms of dispersion coefficients and/or source height.
++
−−
2
z
eH
2
1-exp
2
z
eH
2
1-exp
2
y
y
2
1exp
zyu2
Q=C
σσσσσπ
zz
Considerations for Flare Modeling
Heat Release and Radiation LossEffective Diameter
Emissions – Combustion Products and Unburned Emitted PollutantsHigh Exhaust TemperatureSignificant Buoyancy Plume Rise
Accidental Release ModelsToxic Models
Neutrally buoyant: INPUFF, AFTOX, etc.Dense Gas: DEGADIS, SLAB, or SOURCE5 for LNG source term calculation, etc.
Fire ModelsLFGRISK for LNG including Jet Fire model and Pool Fire model
Considerations for Accidental ReleaseRelease RateRelease DurationRelease Conditions (temperature, pressure, hole/pipe size)Dense gas vs. Neutrally buoyantIdeal gas vs non-ideal gasLiquid vapor equilibrium
Single compound flashingMulti-compound flashing
Single-phase vs two-phase Choked vs non-choked flow
Dense Gas Modeling
Dense gas models consider the non-Gaussian behavior of concentrated, heavier-than-air, releases to the atmosphere.Heavier-than-air releases tend to display three distinct transport regimes
SlumpingGround-huggingPassive dispersion (neutrally buoyant)
Utilizing Modeling to Determine Distance to Level of Concern
Modeled H2S Concentration at Sea Level
050
100150200
0 50 100 150 200 250 300 350 400 450 500Downwind Distance (m)
Con
c (p
pm)
Model Accuracy40 CFR 51 Appendix W – EPA Model Guideline
“Models are more reliable for estimating longer time-averaged concentrations than for estimating short-term concentrations at a specific location.The models are reasonably reliable in estimating the magnitude of highest concentrations occurring sometime, somewhere within an area.Errors in highest estimated concentrations of 10 to 40 percent are found to be typical. Estimates of concentrations that occur at a specific time and site are poorly correlated with actually observed concentrations and are much less reliable.Uncertainties do not indicate that an estimated concentration does not occur, only that the precise time and locations are in doubt.”
Advantages of ModelingCan be used to simulate as many actual or potential “what-if” emission scenarios as necessary.Useful to identify or predict areas of concern due to air emissions.Useful to determine the radius of impact to specific level of concerns for released toxic/flammable compounds.Useful to develop prevention/emergency response plan to reduce risk to potential toxic/fire hazards.Cost-effective
Contact InformationWeiping Dai, Ph.D., P.E. Phone: 972-661-8100Fax: 972-385-9203Email: wdai@trinityconsultants.comAddress: Trinity Consultants
12801 N. Central Expressway, Suite 1200Dallas, TX 75243
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